Amlodipine is a calcium ion antagonist, which is used for clinically treating hypertension and stable angina. Currently, amlodipine clinically used is substantially the raceme thereof. It is reported that the main pharmacological active ingredient in the raceme is (S)-(−)-amlodipine and the antagonistic activity thereof against calcium ion is about 1000 times to that of (R)-(+)-amlodipine and double to that of the raceme per se by Arrowsmiith, J. E. et al., J. Med. Chem. (1986) 29; 1696-1702. WO93/10779 (Young, J. W.) discloses that the administration of (S)-(−)-amlodipine can reduce the side effects such as acroedema, headache, dizziness etc. as compared with using racemic amlodipine. Accordingly, the therapies of hypertension and stable angina by administrating (S)-(−)-amlodipine have a good market prospect. The other enantiomer, (R)-(+)-amlodipine, has the activity of treating atherosclerosis.
The chemical structure formula of amlodipine is showed by

The method for preparing the enantiomers of amlodipine essentially comprises the resolution of racemic amlodipine. WO95/25722 (Pfizer Pharmaceuticals Limited) provides a method for obtaining the enantiomers of amlodipine by direct resolution of amlodipine using D- or L-tartaric acid as resolving agent and dimethyl sulphoxide (DMSO) as solvent, which has a disadvantage in that a high boiling point of DMSO solvent, i.e. 189° C., tends to cause the recovery of solvent is difficult during the preparation. WO 03/035623 (Sepracor Co.) describes a method for directly resolving racemic amlodipine by using D- or L-tartaric acid as resolving agent and N,N-dimethyl acetamide (DMAC) as solvent. Similarly, it is difficult to recover DMAC due to the high boiling point of about 164-166° C. And since DMAC belongs to the Second Solvent which has high toxicity (Guidance for Industry IMPURITIES: RESIDUAL SOLVENTS, FDA, May 15, 2001), it tends to result in the serious environmental contamination during the preparation.